Problems are encountered in the speed control of an elevator when the elevator is moving at a low speed while approaching a landing in order to stop or departing from a landing. The start of the movement of the elevator should be soft and free of jerks. In order to enable the elevator car in particular to start moving in a soft and jerk-free manner, the hoisting motor of the elevator is conventionally controlled using a speed reference adjusted for this purpose and a feedback speed controller. The feedback element used is typically a tachometer which measures the speed from the motor shaft, giving a voltage or pulse frequency proportional to the speed. The feedback element conventionally used in the elevator speed controller is a direct voltage tachometer whose output voltage is directly proportional to the rotational speed of the motor, which can be used to determine the vertical speed of the elevator.
Controlling the elevator speed is a problem when the elevator is moving at a low speed while approaching a landing in order to stop or departing from a landing. In the case of geared elevators, the transition from a static friction condition to a condition where kinetic friction prevails is particularly difficult to manage. The elevator car does not always move as one would expect it to when observing the speed of the motor shaft. The elevator guides, especially sliding guides, may be so tight that, to overcome the static friction at the departure of the elevator, a considerable "extra" motor torque is needed, before the motor shaft starts rotating. This also applies to the hoisting machinery, in which the static friction of the bearings has to be overcome.
The internal friction of the bearings and hoisting machinery is especially significant in geared elevators. A situation readily arises where the speed reference, and often also the speed difference, has become fairly large before the static friction is overcome. It is practically impossible to correct any large vibrations of the elevator car if the correction is based on observing the rotation of the motor shaft. When the elevator car finally starts moving, it is not possible to avoid a jerk being felt in the car by detecting the speed of the motor shaft. This is true especially if, due to rope elongation, energy is stored in the elevator ropes and is then discharged as the static friction is changed into kinetic friction that is lower than the static friction. The problem can be regarded as being based on the absence of correct, sufficiently accurate and timely feedback information about the position and/or motional condition of the elevator car.
When the elevator starts moving, there should be a way to reduce the torque in time from the level needed to overcome the static friction to a level corresponding to the motional condition of the car and the kinetic friction of the system, but as there is no direct information available about the speed level of the car other than a motor speed tachometer signal which cannot consider rope elongation data or other differences prevailing in the system between the tachometer data and the actual motional condition of the car, the motor is likely to maintain the torque corresponding to the static friction longer than necessary. In this way, when the car starts moving, the system readily produces a starting jerk which then continues in the form of decreasing oscillation.
To provide a solution to the problem of a starting jerk and oscillation, an accelerometer placed in the car has been proposed. In this case, the acceleration signal obtained from the accelerometer would be converted into a car speed signal, which would further be used to adjust the car speed instead of the motor shaft speed. However, the accelerometer is an expensive and sensitive component and its output signal requires a high class amplifier to produce a reliable signal.
Conventional start adjustment of an elevator involves the use of an electronic weighing device which measures the torque on the motor shaft via brake shoes. The output of the weighing device is passed to a regulator which controls the motor torque in such a way that it cancels the torque resulting from the load, in other words, the torque acting on the weighing device is adjusted to zero. However, this type of start adjustment requires expensive mechanical brake shoe solutions for the machinery, the weighing device elements are susceptible to damage, and the system must be used as before each time an elevator is used. Additionally the weighing device electronics have to be calibrated to adapt them to the particular elevator.
One of the factors causing problems is the absence of sufficiently correct position data when the elevator is moving near a landing at a low speed, i.e. almost 0-speed. While the tachometer signal does give a fairly good speed data resolution even for low speeds, the position data obtained via calculation from the tachometer signal may clearly differ from the actual position of the elevator car.
To meet the needs and solve the problems described above, an apparatus and a procedure for controlling the hoisting motor of an elevator using positional feedback from a linear position sensor to smoothly overcome static friction during acceleration are presented as an invention.